The present invention relates to a restoration technique in mixed telecommunication networks comprising SONET (Synchronous Optical NETwork) rings and WDM (Wavelength Division Multiplexing) point-to-point links.
Wide-scale deployment of WDM systems carrying many independent traffic channels (8/16/32 and more) over the same fiber transformed the ideas of all-optical networking to a reality of telecommunication business. All-optical networking has been a subject of both academic and applied research for over a decade [A. Elrefaie, Self-healing Ring Network Architecture Using WDM for Growth, In ECOC'91, 285, Paper TuPt.16, 1991; F. J. Janniello, R. Ramaswami, and D. G. Steinberg, A Prototype Circuit-Switched Multi-wavelength Optical Metropolitan-area Network, J. Lightwave Tech. 11, 777, 1993]. The viewpoint developed along the lines of this research predicts specific sequence of all-optical networking development. One of the well known strategies suggests that point-to-point WDM systems should be deployed first. This prediction is already confirmed by the industrial development. As a next step, deployment of tunable add-drop multiplexers and eventually of all-optical rings is predicted, in direct analogy with SONET ring architecture. It was also expected that the optical layer services such as optical restoration, wavelength translation etc. will be implemented together with the optical rings. However neither tunable add-drop multiplexers or optical rings are intended for deployment. An opposite tendency exists to use the capability of WDM systems to upgrade the fiber capacity without building additional SONET rings. The point-to-point WDM links allow to increase network capacity but they have no built-in mechanisms for service protection (restoration) in case of fiber or cable cut. For protection purposes, WDM links employ the algorithms of protection acquired from SONET rings.
SONET rings have built-in fiber structure (working and protection fibers) for service restoration. In case of fiber or cable cut, the traffic is switched from the failed working fiber to the protection fiber using electronic (customary) or optical switches. SONET rings are also provisioned for equipment protection, usually on 1+1 basis when for each working network element, a stand-by protecting element is provided.
In mixed SONET/WDM environment, the WDM link may carry traffic of several SONET rings plus traffic added and dropped at WDM terminals. With the existing deployment strategy of WDM systems, the working and protection fibers are connected to working and protecting WDM terminals, respectively, which provides equipment and fiber protection along WDM links. The WDM equipment is thus protected on 1+1 basis, similar to SONET equipment protection. However, WDM links are not adequately protected against cable cuts. When cable is cut (working and protection fibers are broken), all SONET rings passing through the respective WDM link have to switch independently. Thus, the local failure propagates across the network causing, for the number of channels over 16, traffic perturbation on a national scale. Furthermore, in case of cable cut the traffic added and dropped at WDM terminals is lost. Thus, any capacity upgrade provided by WDM link has to be followed by building additional SONET rings which causes additional spending and network complexity due to protection needs only.
Hence, none of the prior art technologies is capable of providing the level of protection for WDM links equivalent to SONET ring protection or sufficient for required network reliability. The present invention suggests to deploy all-optical protection functionality for WDM links before the optical ring architecture is developed.